Muscles Anatomy Physiology

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Muscles Anatomy Physiology
2010-03-10 13:55:19
Muscles physiology

Muscles physiology
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  1. Skeletal Muscle Tissue
    • Attached to bones and skin
    • Striated
    • Voluntary (i.e., conscious control)
    • Powerful
    • Primary topic of this chapter
    • Each muscle is served by one artery, one nerve, and one or more veins
  2. Cardiac Muscle
    • Only in the heart
    • Striated
    • Involuntary
    • More details in Chapter 18
  3. Smooth Muscle Tissue
    • In the walls of hollow organs, e.g., stomach, urinary bladder, and airways
    • Not striated
    • Involuntary
    • More details later in this chapter
  4. Special Characteristics of Muscle Tissue
    • Excitability (responsiveness or irritability): ability to receive and respond to stimuli
    • Contractility: ability to shorten when stimulated
    • Extensibility: ability to be stretched
    • Elasticity: ability to recoil to resting length
  5. Muscle Functions
    • Movement of bones or fluids (e.g., blood)
    • Maintaining posture and body position
    • Stabilizing joints
    • Heat generation (especially skeletal muscle)
  6. Connective tissue sheaths of skeletal muscle:
    • Epimysium: dense regular connective tissue surrounding entire muscle
    • Perimysium: fibrous connective tissue surrounding fascicles (groups of muscle fibers)
    • Endomysium: fine areolar connective tissue surrounding each muscle fiber
  7. Muscle Attachments
    Directly—epimysium of muscle is fused to the periosteum of bone or perichondrium of cartilage

    Indirectly—connective tissue wrappings extend beyond the muscle as a ropelike tendon or sheetlike aponeurosis
  8. Charectoristics of a Muscle Cell
    *Cylindrical cell 10 to 100 micrometers in diameter, up to 30 cm long

    *Multiple peripheral nuclei

    *Many mitochondria

    *Glycosomes for glycogen storage, myoglobin for O2 storage

    *Also contain myofibrils, sarcoplasmic reticulum, and T tubules
  9. Myofibrils
    • *Densely packed, rodlike elements
    • *~80% of cell volume
    • *Exhibit striations: perfectly aligned repeating series of dark A bands and light I bands
  10. Sarcomere
    • *Smallest contractile unit (functional unit) of a muscle CELL
    • *The region of a myofibril between two successive Z discs
    • *Composed of thick and thin myofilaments made of contractile proteins
  11. Features of a Sarcomere
    • *Thin filaments: run the length of the I band and partway into the A band
    • *Z disc: coin-shaped sheet of proteins that anchors the thin filaments and connects myofibrils to one another
    • *Thick filaments: run the entire length of an A band
    • *H zone: lighter midregion where filaments do not overlap
    • *M line: line of protein myomesin that holds adjacent thick filaments together
  12. Ultrastructure of Thick Filament
    • Composed of the protein myosin
    • *Myosin tails contain: 2 interwoven, heavy polypeptide chains
    • *Myosin heads contain:
    • - 2 smaller, light polypeptide chains that act as cross bridges during contraction
    • -Binding sites for actin of thin filaments
    • -Binding sites for ATP
    • -ATPase enzymes
  13. Ultrastructure of Thin Filament
    • *Twisted double strand of fibrous protein F actin
    • *F actin consists of G (globular) actin subunits
    • *G actin bears active sites for myosin head attachment during contraction
    • *Tropomyosin and troponin: regulatory proteins bound to actin
  14. Sarcoplasmic Reticulum (SR)
    • *Network of smooth endoplasmic reticulum surrounding each myofibril
    • *Pairs of terminal cisternae form perpendicular cross channels
    • *Functions in the regulation of intracellular Ca2+ levels
  15. T Tubules
    • *Continuous with the sarcolemma
    • *Penetrate the cell’s interior at each A band–I band junction
    • *Associate with the paired terminal cisternae to form triads that encircle each sarcomere
  16. Triad Relationships
    • *T tubules conduct impulses deep into muscle fiber
    • *Integral proteins protrude into the intermembrane space from T tubule and SR cisternae membranes
    • *T tubule proteins: voltage sensors
    • *SR foot proteins: gated channels that regulate Ca2+ release from the SR cisternae
  17. Contraction
    • *The generation of force
    • *Does not necessarily cause shortening of the fiber
    • *Shortening occurs when tension generated by cross bridges on the thin filaments exceeds forces opposing shortening
  18. Sliding Filament Model of Contraction
    *In the relaxed state, thin and thick filaments overlap only slightly

    *During contraction, myosin heads bind to actin, detach, and bind again, to propel the thin filaments toward the M line

    *As H zones shorten and disappear, sarcomeres shorten, muscle cells shorten, and the whole muscle shortens
  19. Requirements for Skeletal Muscle Contraction
    1. Activation: neural stimulation at aneuromuscular junction

    • 2. Excitation-contraction coupling:
    • -Generation and propagation of an action potential along the sarcolemma

    -Final trigger: a brief rise in intracellular Ca2+ levels
  20. Events at the Neuromuscular Junction
    • *Skeletal muscles are stimulated by somatic motor neurons
    • *Axons of motor neurons travel from the central nervous system via nerves to skeletal muscles
    • *Each axon forms several branches as it enters a muscle
    • *Each axon ending forms a neuromuscular junction with a single muscle fiber
  21. Events at NeuroMuscular Junction
    • 1. Actionpotential arrives at the axon terminal of a motor neauron
    • 2.Voltage Gated Calcium (Ca2) chammels open and the Ca2 enters th axon terminal
    • 3.Ca2+ entry causes some synaptic vesicles to release their contents (acetylcholine)by exocytosis.
    • 4.Acetylcholine, aneurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma.
    • 5.ACh binding opens ionchannels that allow simultaneous passage of Na+ into the musclefiber and K+ out of the muscle fiber.
    • 6.ACh effects are terminated by its enzymatic breakdown in the synaptic cleft by acetylcholinesterase.
  22. Destruction of Acetylcholine
    *ACh effects are quickly terminated by the enzyme acetylcholinesterase

    *Prevents continued muscle fiber contraction in the absence of additional stimulation
  23. Events in Generation of an Action Potential
    • 1.Local depolarization (end plate potential):
    • -ACh binding opens chemically (ligand) gated ion channels
    • -Simultaneous diffusion of Na+ (inward) and K+ (outward)
    • -More Na+ diffuses, so the interior of the sarcolemma becomes less negative
    • -Local depolarization – end plate potential
    • 2.Generation and propagation of an action potential:
    • End plate potential spreads to adjacent membrane areas
    • Voltage-gated Na+ channels open
    • Na+ influx decreases the membrane voltage toward a critical threshold
    • If threshold is reached, an action potential is generated
    • Local depolarization wave continues to spread, changing the permeability of the sarcolemma
    • Voltage-regulated Na+ channels open in the adjacent patch, causing it to depolarize to threshold
    • 3.Repolarization:
    • Na+ channels close and voltage-gated K+ channels open
    • K+ efflux rapidly restores the resting polarity
    • Fiber cannot be stimulated and is in a refractory period until repolarization is complete
    • Ionic conditions of the resting state are restored by the Na+-K+ pump
  24. Role of Calcium (Ca2+) in Contraction
    • At higher intracellular Ca2+ concentrations:
    • Ca2+ binds to troponin Troponin
    • changes shape and moves tropomyosin away from active sites
    • Events of the cross bridge cycle occur
    • When nervous stimulation ceases, Ca2+ is pumped back into the SR and contraction ends
  25. Cross Bridge Cycle
    • Continues as long as the Ca2+ signal and adequate ATP are present
    • Cross bridge formation—high-energy myosin head attaches to thin filament
    • Working (power) stroke—myosin head pivots and pulls thin filament toward M line
    • Cross bridge detachment—ATP attaches to myosin head and the cross bridge detaches
    • “Cocking” of the myosin head—energy from hydrolysis of ATP cocks the myosin head into the high-energy state
  26. Review Principles of Muscle Mechanics
    • 1.Contraction happens in single cells and in whole muscles
    • 2.Contraction produces tension (force)
    • 3.Contraction does not always shorten a muscle
    • 4.Force and duration of contraction vary in response to stimuli of different frequencies and intensities
  27. ATP is regenerated by
    ATP is regenerated by:

    • Direct phosphorylation of ADP by creatine phosphate (CP)
    • Anaerobic pathway (glycolysis)
    • Aerobic respiration
  28. Muscle Cell Types Visually
    • White = Fast glycolytic cells = short fast activity
    • Red = Slow oxidative cells= long slow activity
    • (Pink= Fast oxidative cells)= long and fast!